Nothing will break when a byte has a high bit set, since it will just be copied to the output unchanged.
Don't panic about UTF-8. The biggest problem with it is people who do not understand it, some of them are good enough programmers that they might write some code that is very damaging, where they actually try to interpret the UTF-8 encoding.
The only real bug in Unix with UTF-8 is a whole lot of documentation that says "character" where it should say "byte". There is nothing wrong with the current implementations.